present invention relates to absorbent articles, particularly absorbent structures which are useful in personal care products such as disposable sanitary napkins, diapers, or incontinence guards. More particularly, the invention relates to absorbent articles which have a portion designed for rapid uptake, temporary liquid control, and subsequent release of repeated liquid surges to the remainder of the article.
The desired performance objectives of personal care absorbent products include low or no leakage from the product, a dry feel to the wearer, and thinness as a means to provide comfort to the wearer. Current absorbent products, however, often fail to meet these objectives for a variety of reasons.
Leakage can occur, for example, due to insufficient uptake rate by layers intended to provide retention or distribution capability in the intake or target zone. Attempts to alleviate leakage occurring by this mechanism include absorbent articles that incorporate surge material structures located above (i.e., toward the wearer) the retention or distribution materials. U.S. Pat. No. 5,364,382 to Latimer discloses nonwoven materials such as meltblowns, bonded carded webs, and pulp conforms that receive and subsequently release liquid to the retention means. The material structures of Latimer utilize large denier resilient fibers blended with small denier wettable fibers to achieve rapid liquid uptake and rapid liquid release to the underlying retention storage material. Additionally, U.S. Pat. No. 5,490,846 to Ellis discloses layered structures to improve intake rates of surge materials.
Despite the development of surge materials that attempt to achieve rapid uptake and rapid release to the retention material, the objective of thinness remains to be satisfactorily reached. The cited surges are quite thick and when placed into the intake zone of the absorbent article can cause poor fit in the crotch region of the absorbent product upon initial wearing and can lead to several performance problems. Firstly, the product can leak due to gapping that is created by the bulky surge material. Secondly, the product is not comfortable to the wearer when a bulky material is utilized to provide the necessary void volume for uptake. There remains a need, therefore, for a surge material which will rapidly uptake an insult to the target area and release it for subsequent storage and which also remains relatively thin prior to insult.
It is an object of this invention to provide a surge material for personal care products which rapidly uptakes an insult and transfers it to an adjacent material for distribution or storage, and which remains relatively thin prior to initial insult. It is another object of this invention to provide a personal care product which, prior to insult, is thin and comfortable for a wearer.
The objectives of the invention are achieved by materials and products which have been designed to be very thin prior to insult and expand rapidly when insulted. In its broadest definition the invention is a fibrous web which includes a binder and which is compressed to a density of up to about 0.3 g/cc. The web may preferably be formed by the bonded carded web process, conform process or air lay process. The binder may depend on hydrogen bonding using moisture or may be a non-aqueous solution, a powder, a fibrous binder or a conjugate fiber binder including a moisture triggerable component. Conjugate fibers may be crimped. When wetted the web of this invention should expand rapidly to greater than 80% of its uncompressed thickness and greater than 90% of its thickness when saturated if starting from the uncompressed state.
xe2x80x9cHydrophilicxe2x80x9d describes fibers or the surfaces of fibers which are wetted by the aqueous liquids in contact with the fibers. The degree of wetting of the materials can, in turn, be described in terms of the contact angles and the surface tensions of the liquids and materials involved. Equipment and techniques suitable for measuring the wettability of particular fiber materials or blends of fiber materials can be provided by a Cahn SFA-222 Surface Force Analyzer System, or a substantially equivalent system. When measured with this system, fibers having contact angles less than 90xc2x0 are designated xe2x80x9cwettablexe2x80x9d or hydrophilic, while fibers having contact angles equal to or greater than 90xc2x0 are designated xe2x80x9cnonwettablexe2x80x9d or hydrophobic.
xe2x80x9cLayerxe2x80x9d when used in the singular can have the dual meaning of a single element or a plurality of elements.
xe2x80x9cLiquidxe2x80x9d means a nongaseous, non-particulate substance and/or material that flows and can assume the interior shape of a container into which it is poured or placed.
As used herein the term xe2x80x9cnonwoven fabric or webxe2x80x9d means a web having a structure of individual fibers or threads which are interlaid, but not in an identifiable manner as in a knitted fabric. Nonwoven fabrics or webs have been formed from many processes such as for example, meltblowing processes, spunbonding processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters useful are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91).
As used herein the term xe2x80x9cmicrofibersxe2x80x9d means small diameter fibers having an average diameter not greater than about 75 microns, for example, having an average diameter of from about 0.5 microns to about 50 microns, or more particularly, microfibers may have an average diameter of from about 2 microns to about 40 microns. Another frequently used expression of fiber diameter is denier, which is defined as grams per 9000 meters of a fiber and may be calculated as fiber diameter in microns squared, multiplied by the density in grams/cc, multiplied by 0.00707. A lower denier indicates a finer fiber and a higher denier indicates a thicker or heavier fiber. For example, the diameter of a polypropylene fiber given as 15 microns may be converted to denier by squaring, multiplying the result by 0.89 g/cc and multiplying by 0.00707. Thus, a 15 micron polypropylene fiber has a denier of about 1.42(152xc3x970.89xc3x970.00707=1.415). Outside the United States the unit of measurement is more commonly the xe2x80x9ctexxe2x80x9d, which is defined as the grams per kilometer of fiber. Tex may be calculated as denier/9.
xe2x80x9cSpunbonded fibersxe2x80x9d refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinneret with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. No. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartman, and U.S. Pat. No. 3,542,615 to Dobo et al. Spunbond fibers are generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and have average diameters (from a sample of at least 10) larger than 7 microns, more particularly, between about 10 and 20 microns.
xe2x80x9cMeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity, usually hot, gas (e.g. air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No.3,849,241. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in average diameter, and are generally tacky when deposited onto a collecting surface.
As used herein, the term xe2x80x9cconformxe2x80x9d means a process in which at least one meltblown diehead is arranged near a chute through which other materials are added to the web while it is forming. Such other materials may be wood pulp, superabsorbent particles, cellulose or staple fibers, for example. Conform processes are shown in commonly assigned U.S. Pat. No.4,818,464 to Lau and U.S. Pat. No. 4,100,324 to Anderson et al. Webs produced by the conform process are generally referred to as conform materials.
xe2x80x9cConjugate fibersxe2x80x9d refers to fibers which have been formed from at least two polymer sources extruded from separate extruders but spun together to form one fiber. Conjugate fibers are also sometimes referred to as multicomponent or bicomponent fibers. The polymers are usually different from each other though conjugate fibers may be monocomponent fibers. The polymers are arranged in substantially constantly positioned distinct zones across the cross-section of the conjugate fibers and extend continuously along the length of the conjugate fibers. The configuration of such a conjugate fiber may be, for example, a sheath/core arrangement wherein one polymer is surrounded by another or may be a side by side arrangement, a pie arrangement or an xe2x80x9cislands-in-the-seaxe2x80x9d arrangement. Conjugate fibers are taught, for example, in U.S. Pat. No.5,382,400 to Pike et al. and may be used to produce crimp in the fibers by using the differential rates of expansion and contraction of the two (or more) polymers. Such fibers may also be splittable. Crimped fibers may also be produced by mechanical means and by the process of German Patent DT 25 13 251 A1. For two component fibers, the polymers may be present in ratios of 75/25, 50/50, 25/75 or any other desired ratios. The fibers may also have shapes such as those described in U.S. Pat. No.5,277,976 to Hogle et al. which describes fibers with unconventional shapes.
The methods for making conjugate fibers are well known and need not be described herein in detail. To form a conjugate fiber, generally, two polymers are extruded separately and fed to a polymer distribution system where the polymers are introduced into a segmented spinneret plate. The polymers follow separate paths to the fiber spinneret and are combined in a spinneret hole which comprises either two or more concentric circular holes thus providing a sheath/core type fiber or a circular spinneret hole divided along a diameter into two parts to provide a side-by-side type fiber. The combined polymer filament is then cooled, solidified and drawn, generally by a mechanical rolls system, to an intermediate filament diameter and collected. Subsequently, the filament is xe2x80x9ccold drawnxe2x80x9d, at a temperature below its softening temperature, to the desired finished fiber diameter and is crimped/textured and cut into a desirable fiber length. Conjugate fibers can be cut into relatively short lengths, such as staple fibers which generally have lengths in the range of 25 to 51 millimeters (mm) and short-cut fibers which are even shorter and generally have lengths less than 18 millimeters. See, for example, U.S. Pat. No. 4,789,592 to Taniguchi et al. and U.S. Pat. No. 5,336,552 to Strack et al, both of which are incorporated herein by reference in their entirety
xe2x80x9cBonded carded webxe2x80x9d refers to webs that are made from staple fibers which are sent through a combing or carding unit, which breaks apart and aligns the staple fibers in the machine direction to form a generally machine direction-oriented fibrous nonwoven web. Such fibers are usually purchased in bales which are placed in a picker which separates the fibers prior to the carding unit. Once the web is formed, it then is bonded by one or more of several known bonding methods. One such bonding method is powder bonding, wherein a powdered adhesive is distributed through the web and then activated, usually by heating the web and adhesive with hot air. Another suitable bonding method is pattern bonding, wherein heated calender rolls or ultrasonic bonding equipment are used to bond the fibers together, usually in a localized bond pattern, though the web can be bonded across its entire surface if so desired. Another suitable and well-known bonding method, particularly when using conjugate staple fibers, is through-air bonding.
xe2x80x9cAirlayingxe2x80x9d is a well known process by which a fibrous nonwoven layer can be formed. In the airlaying process, bundles of small fibers having typical lengths ranging from about 6 to about 19 millimeters (mm) are separated and entrained in an air supply and then deposited onto a forming screen, usually with the assistance of a vacuum supply. The randomly deposited fibers then are bonded to one another using, for example, hot air or a spray adhesive.
As used herein xe2x80x9cthermal point bondingxe2x80x9d involves passing a fabric or web of fibers to be bonded between a heated calender roll and an anvil roll. The calender roll is usually, though not always, patterned in some way so that the entire fabric is not bonded across its entire surface, and the anvil roll is usually flat. As a result, various patterns for calender rolls have been developed for functional as well as aesthetic reasons. Typically, the percent bonding area varies from around 10% to around 30% of the area of the fabric laminate web. As is well known in the art, the spot bonding holds the laminate layers together as well as imparts integrity to each individual layer by bonding filaments and/or fibers within each layer.
As used herein, through-air bonding means a process of bonding a fiber web in which air which is sufficiently hot to melt the polymers of which the fibers of the web are made is forced through the web. The air velocity is between 100 and 500 feet per minute and the dwell time may be as long as 6 seconds. The melting and resolidification of the polymer provides the bonding.
xe2x80x9cPersonal care productxe2x80x9d means diapers, training pants, absorbent underpants, feminine hygiene products and adult incontinence products.